1) Field of the Invention
The present invention relates particularly to an apparatus and method for controlling a clutch of a mechanical automatic transmission which is suitable for use in controlling a clutch at the time a vehicle starts to move (hereinafter may be called “at the time of start”).
2) Description of the Related Art
A vehicle—which is equipped with a mechanical automatic transmission arranged to enable automatic transmission by providing a change gear mechanism and a clutch mechanism respectively with an actuator—performs clutch partial engagement control operation at the time of start (or when the vehicle moves slowly). As shown in, e.g., FIG. 8, the clutch partial engagement control is performed on the basis of a map of a clutch stroke speed with respect to a rate of change in an engine speed (i.e., a differential value of the engine speed).
For instance, the rate of change in engine speed is C1 (C1>0) , the clutch is controlled so as to move toward an engagement direction at a stroke speed V1. When the rate of change in engine speed is C2 (C2<0), the clutch is controlled so as to move toward a disengagement direction at a stroke speed V2. When the rate of change in engine speed is zero, the clutch is controlled so as to stay at the current position.
As a result, at the time of start of the vehicle, clutch partial engagement control operation is commenced at a point in time when a driver has stepped on an accelerator, and the clutch is gradually engaged, to thus accelerate the vehicle.
JP-A-6-117454 (hereinafter called “Patent Publication 1”) describes a technique for computing a target engine speed at which a clutch is to be thrown in for smoothly driving a vehicle, from throttle opening achieved at the time of start, thereby increasing or decreasing an exciting current for an electromagnetic powder clutch such that an actual engine speed attains the target engine speed.
In a diesel engine, engine output torque corresponding to an accelerator position VA (%) set by the driver has a characteristic with respect to the engine speed such as that shown in FIG. 9.
As shown in FIG. 9, for example, when the accelerator position is 0%, output torque assumes a value of 0 at an idle speed of 650 rpm. When the idle speed has increased to become higher than the engine speed, the engine output torque gradually decreases.
For instance, when the accelerator position is 10%, the higher the engine speed, the higher the engine output torque. However, the engine output torque has reached a peak (maximum torque) at a certain engine speed P10, the engine output torque gradually decreases in subsequent operation. Similarly, even when the accelerator position is 20%, the higher the engine speed, the higher the engine output torque. However, the engine output torque reaches a peak at a certain engine speed P20, and the engine output torque gradually decreases in a subsequent operation.
As mentioned above, the diesel engine has a parabolic characteristic, wherein, as the accelerator position increases, the peak of the engine speed shifts to a higher range, and the engine output torque gradually decreases after the engine speed has exceeded the peak.
Therefore, when the clutch partial engagement control operation is performed at the time of start of a vehicle without taking into consideration such an output torque characteristic, the clutch may be partially engaged while the engine speed remains below the engine speed achieved at the peak (the maximum torque output) (i.e., a drop has arisen in engine speed) or higher than the engine speed achieved at the peak (i.e., a hike has arisen in engine speed).
In this case, even when clutch partial engagement operation has been performed, sufficient drive torque cannot be output, because the engine output torque is too low, thus posing difficulty in smooth start of the vehicle. Particularly, this problem becomes noticeable when the vehicle starts moving while on an uphill or when a vehicle with a heavy load starts moving.
In association with occurrence of such a problem, the time which lapses before the vehicle starts moving becomes longer; that is, the time during which the clutch is partially thrown in becomes longer, and hence the clutch may become abraded early, or noise due to an excessive hike in the engine speed may increase.
According to the previously-described technique of Patent Document 1, the engine speed at which maximum torque can be achieved is not set as a target rotational engine speed at all times. For this reason, when the vehicle is under heavy load, difficulty is encountered in smoothly starting the vehicle. Moreover, the clutch described in Patent Document 1 is of electromagnetic power type, wherein no attention is paid to abrasion developing in the clutch.